1. Field of the Invention
This invention relates to a sub-Nyquist sampling encoder and decoder used for a transmission/recording apparatus of digitalized video signals such as a digital VTR.
2. Description of the Related Art
A typical example of a conventional sub-Nyquist sampling encoder and decoder will first be described with reference to FIGS. 1, 2, 3A and 3B. Referring to FIG. 1, a recording/transmission system based on sub-Nyquist sampling comprises an A/D converter 1 for sampling an analog pixel signal at a frequency fs, a pre-filter 2 for band width limitation of a spatial frequency of the video signal, a recording/transmission medium 3, a post-filter 4 for interpolating thinned-out pixels, and a D/A converter 5 for converting a sampled pixel string into an analog video signal. Pixels sampled by the A/D converter 1 are patterned into a lattice configuration on the screen, as shown in FIG. 2, and the number of pixels is halved by a thin-out operation. The thin-out operation is described as down-sampling in FIG. 1 and carried out to thin out pixels as depicted by a smaller hatched circle from the sampled pixels in FIG. 2, leaving behind pixels as depicted by a larger blank circle. The down-sampling for an upper line is offset, as illustrated, from that for a lower line with the view of conserving horizontal resolution and vertical resolution in the horizontally and vertically defined two-dimensional frequency space of the video signal before and after the down-sampling. In order to minimize interpolation errors as a whole, it is necessary to effect the band width limitation by means of the pre-filter in advance of the down-sampling operation. The pre-filter 2 in the FIG. 1 example has a filter characteristic as shown in FIG. 3A by which a stringent band width limitation is imposed on frequency components in the skewed direction. The pixel string is passed through the transmission/recording medium at a clock rate of 1/2 fs and then filled, at pixel positions which are scheduled to be interpolated, with pixels of zero value. This operation is called up-sampling and allows the pixel string to match the clock rate fs. The post-filter acts on the resulting pixel string to perform interpolation. The post-filter 4 in the FIG. 1 example has a filter characteristic as shown in FIG. 3B and functions as a two-dimensional low-pass filter having values used as interpolation values.
The pixel string thus interpolated at the post-filter is passed through the D/A converter 5 and delivered out.
In the conventional construction as above, however, the band width limitation imposed by the pre-filter 2 and post-filter 4 is not sufficiently stringent and accordingly the horizontal resolution and vertical resolution are disadvantageously degraded.
In order to make ideal the transfer characteristics of the pre-filter and post-filter, it is conceivable to increase the degree of these filters but this expedient is expected to lead to a increase in circuit scale and an increase in cost.